The general field of the present invention is the interconnection of ceramic members to metallic members.
Currently ceramics are increasingly being used in place of metals in mechanical environments where heat resistance and/or low specific gravities are important. For example, ceramic turbine disks are replacing metal disks in the turbo-superchargers of internal combustion engines. The turbo-supercharger is a unit for pre-compressing intake air or air/fuel mixture. The supercharger makes use of the pressure generated by exhaust gases to turn an exhaust turbine disk which drives a compressor. Since the turbine disk is a member which is exposed to hot exhaust gases and which rotates at high speeds, it is more efficient to make the disk of ceramics which have higher heat resistance and lower specific gravities than metals. Moreover, certain ceramics are as strong as metals.
On the other hand, metals are still used for certain purposes and therefore a means of joining the ceramic parts to the metal parts must be found. The problem is the thermal expansion coefficients of the metals used tend to be two to five times higher than those of ceramics. For example, Cr-Mo steel has a coefficient of thermal expansion of E=11.7.times.10.sup.-6 /.degree. C. whereas the ceramic, silicon nitride (Si.sub.3 N.sub.4), has a coefficient of thermal expansion of E=2.6 to 4.5.times.10.sup.-6 /.degree. C. In high temperature environments, metal parts thus have a tendency to expand more rapidly than the ceramic parts, often causing the metal to pull away from or break the ceramic.
Various means for joining ceramics to metals have been proposed. Examples are described in Japanese Patent Application Laid-Open No. 103902/1984 and Japanese Utility Model Application Laid-Open No. 5701/1984, the disclosures of which are incorporated herein by reference and are briefly discussed in the Detailed Descriptions of the Drawings of this invention. In the above disclosures, a boss portion of a ceramic disk extends into a cup-shaped end of a metal shaft. The boss portion is joined by a shrink-fit connection or by brazing to the sleeves of the cup-shaped end which sleeves are made of metal with a thermal expansion coefficient that is substantially the same as that of the ceramic. Since the ceramic boss and the metallic sleeve expand at a nearly equal rate, normally the connection between the boss and the sleeve is not broken at high temperatures. However, other cracks and failures can occur in these prior art arrangements.